Electronic devices are continuously offering more functionality in smaller packages. This is enabled, in part, by integrating more capability—processing power, memory, etc.—onto individual integrated circuit chips. However, also important in the development of small, powerful devices is the ability to fit more of the integrated circuit chips themselves into smaller packages.
Integrated circuit chips are typically attached to printed circuit boards. These boards contain one or more layers of metal traces and vias, providing electrical connections to chips and other components, thus completing the electronic system. By using innovative ways of attaching their component chips, boards can be made smaller in order to fit into smaller devices.
Integrated circuit chips can be attached to printed circuit boards in several ways. Often they are mounted in packages that have various configurations of pins, which, in turn, are inserted into holes in the printed circuit boards and fixed in place. For a smaller outline, the packaging step can be omitted, and the chip can be mounted directly on the board. A common chip mounting technique—for mounting chips both in packages and directly on boards—is wire bonding. In this method, thin wires connect pads in the package, or on the board, to pads on the chip. Usually, these bonding pads lie along the outside edges of the upper surface of the chip.
Since the board area needed for a wire-bonded chip exceeds the chip area by the length of the wires, other methods are available to replace wire bonding. In a second method, known as flip-chip or C4 (for controlled collapse chip connection), bond pads on the chip are coated with solder bumps, and the chip is mounted face down on the board. In this method, the footprint on the board used by the chip is no larger than the area of the chip. Eliminating the long wires may have performance advantages as well.
Another method of reducing board size is to stack chips on top of each other, while still being electrically connected to the board. Designers often find it advantageous to stack related chips—for example, a memory chip and its controller. In this case, the upper chip is usually connected directly to lower chip, and not necessarily to the board. Such a stacked chip assembly will typically require a vertical connection, such as a through-silicon via, to route signals and/or power to at least one of the chips. Such vertical connections, though expensive, can result in substantial package size reductions, especially if this technique is combined with flip-chip mounting. In these assemblies, both chips are either upside down, with C4 bumps formed on the lower chip; or they are mounted face-to-face, with the C4 bumps formed directly on vertical connectors.
In some cases, chip stacking may be beneficial but vertical connections are not required. For example, multiple identical memory chips may be connected to one controller chip, so as to increase memory capacity. In this case, the memory chips could be stacked and bonded individually to the printed circuit board, connecting them to the nearby controller chip. In these cases, both chips are typically mounted right side up, and both are wire bonded to the board. However, some of the area savings afforded by chip stacking is lost due to the area consumed by the multitude of wire bonds.
Thus, there is an increasing need to produce small, complex circuit boards in a cost-efficient manner.
As used herein and in the appended claims, the region in which circuitry is formed on a substrate is referred to an active layer. The circuitry referred to by the term “active layer” need not contain any active devices; rather, such a layer may contain a circuit comprising only passive devices. Examples of such passive circuits include bandpass filters and resistor dividers.